Generally, an automatic transmission achieving more speed stages has been developed to enhance fuel economy and optimize drivability. Recently, an increase of oil prices has triggered competition in enhancing fuel consumption of a vehicle.
Therefore, much research for reducing weight and enhancing fuel economy through engine downsizing and for securing drivability and fuel economy through multiple speed stages of automatic transmissions has been undertaken.
However, in the automatic transmission, as the number of speed stages increases, the number of internal components (particularly, planetary gear sets) increases, and as a result, a length of the transmission increases. Therefore, mountability, cost, weight, transmission efficiency, and the like may deteriorate.
Accordingly, development of a planetary gear train which may achieve maximum efficiency with a small number of components may be important in order to increase a fuel economy enhancement effect through the multiple-speeds.
In this aspect, in recent years, 8-speed automatic transmissions have been implemented and the research and development of a planetary gear train capable of implementing more speed stages has also been actively conducted.
However, a conventional 8-speed automatic transmission typically includes three to four planetary gear sets and five to six control elements (frictional elements). In this embodiment, since the length of the automatic transmission may be large, mountability may be deteriorated.
One planetary gear set is often disposed above another planetary gear set, but structures of automatic transmissions to which parallel planetary gear sets are applied are limited.
In another method, dog clutches instead of control elements of a wet-type are used. However, in such an arrangement, shift feel can deteriorate.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.